1. Field of the Invention
The present invention relates to a highly nonlinear optical fiber and a highly nonlinear optical fiber module, and more particularly, to a highly nonlinear optical fiber having a large nonlinear coefficient for optical signal processing making use of nonlinear phenomena and a highly nonlinear optical fiber module serving as a highly nonlinear device that houses the highly nonlinear optical fiber wound in a coil shape.
2. Description of the Related Art
A transmission rate per wavelength channel of optical fiber communication currently put to practical use is 10 Gbit/s. To increase a total transmission capacity without excessively complicating a system in large capacity wavelength division multiplexing (WDM) transmission, it is desirable to increase a transmission rate per channel. Under such circumstances, researches concerning very high-speed optical fiber transmission with a transmission rate of 40 Gbit/s per channel or more have been actively carried out.
In response to the increase in the transmission rate, peak power of a signal increases, and nonlinear phenomena (such as self phase modulation, mutual phase modulation, and four-wave mixing) in an optical fiber appear more conspicuously as peak power of a signal increases. Such nonlinear phenomena cause deterioration in a transmission characteristic. On the other hand, the nonlinear phenomena have an advantage that high-speed responsiveness thereof can be applied to high-speed optical signal processing.
In recent years, a highly nonlinear optical fiber (HNLF) having high nonlinearity has been developed. Following the development, optical signal processing using the highly nonlinear optical fiber is gaining a popularity.
The highly nonlinear optical fiber is used for optical signal processing making use of the nonlinear phenomena. The highly nonlinear optical fiber is not used as a transmission line but formed as a package and incorporated in a transmission apparatus or a light source. When an optical fiber is formed in the package, the highly nonlinear optical fiber is wound around a bobbin or wound annularly without using a bobbin and used.
A volume, which the optical fiber wound in a coil shape in this way occupies in the package, depends on a volume and a percentage of voids of the optical fiber. The percentage of voids is a percentage of a volume, which the optical fiber itself occupies, in a volume V of a portion where the optical fiber is wound. The volume V is represented by the following Eq. (1) in a bobbin shown in FIG. 6. The percentage of voids increases as a cross section of the optical fiber, that is, an outer diameter (hereinafter referred to as coating diameter) of the optical fiber increases.V=W×{((d1−d2)/2)2−(d2/2)2}×π  (1).
When a winding diameter of the optical fiber (an inner diameter of the bobbin) is reduced for the purpose of reducing a size of the package, winding distortion of the optical fiber increases. The winding distortion is proportional to a diameter of a cladding (hereinafter referred to as cladding diameter) of the optical fiber. Therefore, it is necessary to reduce the cladding diameter to reduce the winding distortion.
Conventionally, as a highly nonlinear optical fiber with a reduced diameter, an optical fiber with a cladding diameter of 110 micrometers and a coating diameter of 150 micrometers and an optical fiber with a cladding diameter of 89 micrometers and a coating diameter of 115 micrometers have been proposed (see, for example, U.S. Pat. No. 6,661,958).
When the cladding diameter of the highly nonlinear optical fiber is reduced, the winding diameter thereof decreases. However, for example, this makes it difficult to connect the highly nonlinear optical fiber with an optical fiber having a general cladding diameter like an optical fiber for connecting an inspection apparatus. Note that a most general cladding diameter of optical fibers is about 125 micrometers and a coating diameter thereof is about 250 micrometers.
To guarantee a quality of the optical fiber, the optical fiber undergoes inspections concerning five to ten items like a transmission loss and a dispersion characteristic after manufacturing. The highly nonlinear optical fiber also undergoes the same inspections.
When, for example, an optical time domain reflectometer (OTDR) for measuring a transmission loss is used as an inspection apparatus, an optical fiber to be inspected is connected to the OTDR through the optical fiber for connecting an inspection apparatus.
As a method of connecting optical fibers each other, there are a fusion splicing method and a butt joint method. In the fusion splicing method, in a state in which coatings at connection ends of an optical fiber to be inspected and an optical fiber for connecting an inspection apparatus are removed, fiber end faces of both the optical fibers are heated and fused while optical axes thereof are fit. It is possible to decrease a connecting loss by using this method. However, the fusion requires time and labor.
On the other hand, in the butt joint method, the coatings at the connection ends of both the optical fibers are removed to expose glass sections, and end faces of the glass sections are butted against each other on a V groove to be connected. In this butt joint method, it is possible to connect the optical fibers in a short time.
It is not easy to connect the highly nonlinear optical fiber with a small diameter and the optical fiber for connecting an inspection apparatus with a general diameter on a V groove because cladding diameters of both the optical fibers are different.